With the discovery of conducting polymers in the late 1970's, the possibility of combining the important electronic and optical properties of semiconductors and metals with the attractive mechanical properties and processing advantages of polymers was proposed. Without exception, however, the initial conducting polymer systems were insoluble, intractable, and non-melting (and thus not processible) with relatively poor mechanical properties.
Success in developing a number of processing routes has opened the way to fabricating conductive polymers into fibers and films etc.; i.e. into shapes that can be used for a wide variety of potential applications in the electrical industry. Important problems remain to be solved before such materials can be put into wide spread use. Among these are cost, environmental stability, and melt processibility:
(1) In many cases the starting materials and the synthesis of conjugated polymers are (at least currently) expensive. PA1 (2) Although a number of conjugated polymers have been developed which appear to be stable, long term stability in different environments is still an important problem. PA1 (3) The development of melt processible conducting polymer systems would open a large market for such materials.
One important route toward solution of these problems is to utilize blends and/or composites of conducting polymers with conventional (melt processible) polymers. The dilution of the expensive conjugated polymer in a relatively low cost host (such as, for example polyethylene) immediately yields a major cost benefit. At the same time, such blends effectively encapsulate the conductive polymer fraction within the environmentally stable host, leading to greatly improved long term stability. The achievement of melt-processibility of conductive polymers remains as an important unsolved problem.
There are a large number of melt-processible polymers many of which are commercially available. Today, most melt-processible polymers (such as polyolefins, polyesters, nylons, etc.) with useful mechanical properties are insulators. It would clearly be desirable to render such materials conducting. Previous attempts to render such materials conducting have utilized the general method of filling them with a volume fraction of conducting material such as particles of carbon black, or metal flakes or particles (for example, silver flakes). Addition of such fillers at sufficiently high quantity to yield connected conducting paths (i.e. to be above the percolation threshold; for example, typically about 16% by volume for globular particles) yields moderate electrical conductivities, but at the expense of the mechanical properties. The tensile strength and elongation at break are severely reduced by the fillers. Moreover, because of the sharp onset (known as the "percolation threshold") of the electrical conductivity as a function of the volume fraction of the filler in such conventionally filled polymers, it is difficult to fabricate articles of moderate electrical conductivity (e.g. less than 10.sup.-4 S/cm) for a variety of uses which include the dissipation of static charge. The same disadvantages (e.g. the existence of a sharp threshold for conductivity of the blends at about 16% volume fraction of the conducting polymer, etc.) are relevant to using conducting polymers as filler (see, for example, Conducting Polymer Composites of Soluble Polythiophenes in Polystyrene by S. Hotta, S. D. D. V. Rughooputh and A. J. Heeger, Syn. Mtls. 22(1):79 (1987); and references therein).
Alternatively, reticulate doped polymers (J. K. Kreska, J. Ulanski, and M. Kryszewski, Nature, 298:390 (1981)) are composites with a crystalline additive organized into different structures; i.e. isotropic 3- or 2-dimensional dendritic networks (J. Ulanski, A. Tracz and M. Kryszewski, J. Phys. D: Appl. Phys. 18:L167 (1985); J. K. Jezka, A. Tracz, J. Ulanski and M. Kryszewski, Pol. Patent No. 138,395 (1985)), quasi-one-dimensional highly oriented crystalline `strings` (L. Burda, A. Gracz, T. Pakula, J. Ulanski, and M. Kryszewski J. Phys. D: Appl. Phys. 16:1737 (1983); J. Ulanski, A. Tracz, E. El Shafee, G. Debrue and R. Deltour, Synth. Met. 35:221 (1990)), or chaorically dispersed, separated needles or dendrites [M. Kryszewski, J. K. Jeszka, J. Unanski and A. Tracz, Pure and Applied Chemistry, 56:355 (1984)). Miscellaneous conducting reticulate doped polymers have been obtained using various polymer matrices and various conducting charge-transfer complexes (see J. Ulanski, A. Tracz, J. K. Kreszka and M. Kryszewski, Mol. Cryst. Liq. Cryst. 118:443 (1985)). However, the reticulate doped polymers are obtained by solution casting of films from a common solution of polymer and the charge transfer salt additive in such conditions that during film solidification, the additive crystallizes in situ (under controlled conditions) in the polymer matrix. Although these reticulate doped polymer composite systems exhibit a low percolation threshold and relatively high conductivity, they are not melt-processible.
Thus, the ability to melt-process conducting articles from polymers and thereby to fabricate such conductive polymers by injection molding, film extrusion etc. into shaped articles with excellent mechanical properties remains seriously limited.